九州大学大学院総合理工学報告第 46 巻 第 1 号 19-25 頁 令和 6 年 9 月

Engineering Sciences Reports, Kyushu University
Vol. 46, No. 1, pp. 19-25, SEP, 2024

Photochemical Removal of CH4 in Air Using a
172 nm Xe2 Excimer Lamp at Atmospheric Pressure
Masaharu TSUJI,*1,2† Naohiro KAMO,*3 Masato MIYANO*3
and Masashi KAWAHARA*3
E-mail of corresponding author: tsuji@cm.kyushu-u.ac.jp†(Received June 20, 2024, accepted July 17, 2024)
The photochemical removal of methane was investigated in N2 or air using a head-on or side-on
type 172 nm Xe2 excimer lamp at atmospheric pressure. No photolysis of CH4 was observed in N2
because CH4 does not absorb 172 nm photons. On the other hand, 34% of CH4 (1000 ppm) was
decomposed in air after 30 min photoirradiation using the head-on lamp. When the side-on type lampwith a larger irradiation area was used, the removal rate of CH4 was increased by a factor of about 6.
In the 172 nm photolysis of CH4 in air, not only the O(3P,1D) + CH4 reactions but also the O3 + CH4
reaction can take part in the initial oxidation of CH4. The contribution of O(1D) was examined by
reducing the total pressure to suppress the collisional quenching of O(1D) by the buffer N2/O2 gases.
The contribution of O3 was studied by observing the O3 + CH4 reaction. It was found that the O(1D) +CH4 reaction plays a major role in the initial stage of the oxidation of CH4 under 172 nm VUVirradiation. Major oxidation pathways from CH4 to CO2 under 172 nm VUV photolysis in air are
discussed.
Key words: VUV photolysis, 172 nm Xe2 excimer lamp, CH4, O(1D) atoms, O3, Oxidation, Air,

Atmospheric pressure

1.

Introduction

The concentration of CH4 in the atmospherein 2022 is 1.91 ppm, which is lower than that of
CO2 (417 ppm) by a factor of 218.1-4) However,
the global warming potential (GWP) of CH4 is
84–87 on a 20-year scale and 27–36 on a 100year time scale in comparison of the standard
value for CO2 (GWP = 1).4) The much larger
GWP values of CH4 dominantly originate fromits shorter lifetime than that of stable CO2 in
the atmosphere. The large GWP values of CH4
indicate that CH4 traps far more heat than CO2
in the atmosphere even though its
concentration is low. Actually, recent studies
have shown that CH4 contributes about 0.5 oC(or nearly half) of the observed 1.1 oC presentday warming above the pre-industrial
temperature.4) Therefore, it is highly desirable
to develop a new removal technique of CH4
without using expensive metallic catalysts.
*1

Institute for Materials Chemistry and Engineering,and Research and Education Center of GreenTechnology*2 Department of Applied Science for Electronics andMaterials*3 Department of Applied Science for Electronics andMaterials, Graduate Student

We have recently used vacuum ultraviolet(VUV) excimer lamps to remove NO, NO2, N2O,
SO2, C6H6, CH3CHO, and C2H3CHO.5-16) They
could efficiently be decomposed and oxidized in
air at atmospheric pressure without using any
expensive metallic catalysts. In the present
study, we attempted to use a 172 nm Xe2
excimer lamp as a new photochemical removal
method of CH4 operating at room temperature
without using any catalysts. The major purposeof this study is to obtain fundamental
information on photochemical decomposition of
CH4 under 172 nm VUV irradiation and theclarification of dominant active species foroxidation of CH4. Not only direct VUVphotolysis of CH4, but also reactions of O(3P,1D)
and O3 with CH4 may contribute to the removalof CH4. To examine effects of direct VUVphotolysis, CH4 was decomposed in N2
atmosphere, whereas the contribution of O(1D)
was examined by reducing the total pressure to
suppress the collisional quenching of O(1D) by
the buffer N2/O2 gas mixtures. The contributionof O3 was studied by observing the O3 + CH4
reaction in the same apparatus. On the basis ofabove experiments, dominant active species forthe removal of CH4 under 172 nm photolysis
are determined and major oxidation processesPhotochemical removal of CH4 in air using a 172 nm Xe2 excimer lamp at atmospheric pressure

2.

Experimental

VUV photolysis apparatus used in this work
was the same as that reported previously.12,13)Lights from an unfocused head-on type or sideon type 172 nm Xe2 lamp was used to remove
CH4 at room temperature. Most experiments
were conducted using the head-on lamp (50mW/cm2, 155200 nm range, UER20H172;
Ushio Inc).
Experiments were carried out in a closed
batch system. The total pressure wasmaintained at atmospheric pressure and theCH4 concentration diluted in an N2/O2 mixture(20% O2) was 1000 or 730 ppm in mostexperiments. It was introduced through mass
flowmeters. Sample gases before and after
photoirradiation were analyzed by a HORIBA
gas analysis system (FG122-LS) equipped withan FTIR spectrometer and an ANELVA gas
analysis system (M-200GA-DTS) equippedwith a quadrupole mass spectrometer. A lowsensitivity mass spectrometer was used for thedetermination of the N2/O2 ratio of buffer gases,
whereas a high-sensitivity FTIR system wasused for the detection of CH4, O3, CO, and CO2.
The light path length and the volume of theanalyzing chamber in FTIR were 2.4 m and 300
cm3, respectively. The IR spectra were
measured in the 10004000 cm-1 region with an
optical resolution of 4 cm-1. The concentrationsof CH4, CO, and CO2 were calibrated using
standard samples supplied from gas companies.
The concentration of O3 was evaluated byreference to the standard spectral data
supplied by HORIBA.
We have also studied the O3 + CH4 reaction
at room temperature using the same apparatus.
O(3P,1D) and O3 were initially prepared fromthe 172 nm photolysis of air in the photolysis
chamber. After O3 was partly stored in the
FTIR cell for gas analysis, the photolysis
chamber was evacuated in vacuum. Then O3
was introduced again to the photolysis chamber.
Under such conditions, short lived O(3P,1D)
atoms disappeared and only long-lived O3
molecules survived as an active species. Afterthen a CH4/N2 mixture was introduced to thechamber, and the reactants and products of theO3 + CH4 reaction were monitored online usingthe
FTIR
spectrometer.
The
initialconcentrations of CH4, O2, and O3 in a CH4/air
mixture was 1000 ppm, 20%, and 1.1%,

respectively, and the total pressure was 50 kPa.
The following gases were used without
further purification: N2 (Taiyo Nissan Inc.:
purity >99.9998%), O2 (Nippon Sanso Inc.:>99.99995%), CH4 (Nippon Sanso Inc.: 3000
ppm in high purity N2). CH4 was diluted in N2
or air (20% O2) before use.

3.

Results and discussion

3.1

An attempt of CH4 removal in N2

N2 has no absorption at 172 nm.18,19)
Therefore, only CH4 may absorb 172 nm
photons in a CH4/N2 mixture. In order toexamine the contribution of direct 172 nm VUVphotolysis of CH4, CH4 removal was studied in
N2. Figure 1(a) shows an FTIR spectrum of CH4(1000 ppm) in N2 before photoirradiation,
where two strong CH4 peaks are observed in
the 1200−1400 and 2800−3200 cm-1 regions.
The variation of the main CH4 peak intensity,
which is proportional to the CH4 concentration,
was monitored as a function of irradiation time
0.5

(a)

0.4
Absorbanceof CH4 to CO and CO2 are discussed. All
reported reaction rate constants used for
discussion17) have been measured at 298 K.

0.3
CH4

0.2
0.1
0
4000 3500 3000 2500 2000 1500 1000
-1

Wavenumber / cm

(b)
1000
Concentration of CH4 / ppm

20

800
600
400
200
0

0

5

10

15

20

25

30

Irradiation time / minFig. 1. (a) FTIR spectrum of CH4 in N2
before photoirradiation. (b) dependence ofconcentration of CH4 on the irradiation time.

令和 6 年度

九州大学大学院総合理工学報告

in the 0−30 min range. The CH4 concentration
was essentially independent on the irradiation
time as shown in Fig. 1(b). This shows that CH4
is not decomposed under 172 nm VUVphotoirradiation. According to reported
photoabsorption spectrum of CH4, CH4 hasessentially no absorption (< 10-20 cm2) at 172
nm.20) Little change in the concentration of CH4
after photoirradiation for 30 min is consistent
with the absence of absorption of CH4 at 172
nm.
3.2

CH4 removal in air

The photolysis of CH4 at 172 nm in air was
examined in an N2/O2 mixture at an O2
concentration of 20%. As an example, Figs. 2(a)and 2(b) show FTIR spectra of CH4 (1000 ppm)
before and after 30 min photoirradiation,
respectively. Before photolysis, two CH4 peaks
are observed as in the case of N2 atmosphere.
After photolysis for 30 min, these CH4 peaks

第 46 巻第1号

21

decrease in intensity by 34% and new strong
CO2 and O3 peaks and a very weak CO peak areobserved. Figure 3 shows the concentrations ofCH4, CO, CO2, and O3 as a function of theirradiation time. The concentration of CH4
decreases to 655 ppm almost linearly withincreasing the irradiation time until 30 min.
The concentration of CO increases to 57 ppm
until 15 min and then slowly decreases to 46
ppm in the 1530 min range. The concentrationof CO2 increases almost linearly to 269 ppm
until 30 min. The concentration of CO2 ishigher than that of CO in all time range andthe [CO2]/[CO] concentration ratio increases
from 1.4 to 5.9 with increasing the irradiation
time from 5 min to 30 min. These time profiles
suggest that CO is slowly converted to CO2
under
the
operation
condition.
The
concentration of O3 rapidly increases to 1.3%
until 15 min irradiation and then decreases to
1.1% in the 15−30 min range.

800

1.2
1.0
0.8

CH4

600

CO

0.6

CO2

400

O3 / %

Concetrations of CH4, CO, and CO2 / ppm

1.4
O3

1000

0.4
200
0

0.2
0

5

10

15

20

25

30

0.0

Irradiation time / minFig. 3. Dependence of concentrations of CH4,
CO, CO2, and O3 on the irradiation timemeasured using a head-on lamp.

Fig. 2. (a) FTIR spectrum of CH4 in air (a) at
a total pressure of 100 kPa before and (b)after photoirradiation for 30 min.

CH4 (730 ppm) removal in air was alsostudied by using a side-on lamp. The result
obtained in the 060 min range is shown in Fig.
4. The concentration of CH4 linearly decreases
to 89 ppm until 25 min and slowly decreases tozero at 45 min. The concentration of CO
increases to 158 ppm in the 02 min range andthen slowly decreases to zero in the 245 minrange. The concentration of CO2 rapidly
increases to 350 ppm until 25 min and slowly
increases to 402 ppm in the 25−60 min range.
These time profiles of CO and CO2 indicate that
CO is converted to CO2 during photolysis. The
concentration of O3 rapidly increases to 3.2% inPhotochemical removal of CH4 in air using a 172 nm Xe2 excimer lamp at atmospheric pressure

3.5

800

3.0
CH4

600

2.5

CO
CO2

500

Although the reaction of O(3P) with CH4 isextremely slow, that of O(1D) with CH4 is fast.

2.0

400

1.5

300

O(1D) + CH4 → products(6)(k5 = 1.50  10-10 cm3 molecule-1 s-1)17)

1.0

200

O3

100
0

10

20

30

40

50

0.5
0.0
60

Irradiation time / minFig. 4. Dependence of concentrations of CH4,
CO, CO2, and O3 on the irradiation timemeasured using a side-on lamp.
the 0−4 min range and decreases to 0.98% in
the 460 min range. The decomposition rate ofCH4 by using the side-on lamp (0.077 min-1) is
6.0 times faster than that by using the head-on
lamp (0.0128 min-1) in the 0−30 min range. The
window area of the side-on lamp, from which
VUV enters into the photolysis chamber, is 9.8
times wider than that of the head-on one.12,13)
Therefore, the fraction of photoirradiation
volume in the reaction chamber at 20% O2
increases from 4.3% to 33.3% by changing
excimer lamp from the head-on type to the sideon one.12) This is a major reason for thesignificant enhancement of removal rate of CH4
using the side-on lamp.
3.3

O(3P) + CH4 → OH + CH3(5)(k4 = 5.37  10-18 cm3 molecule-1 s-1)17)

Thus, the O(1D) + CH4 reaction (6) will be an
important reaction for CH4 removal. However,quenching reactions of O(1D) by N2/O2 gases (2)and (3) compete with the O(1D) + CH4 reaction(6). To reduce the relative contribution ofreactions (2) and (3) to that of (6), we attempted
to enhance the decomposition of CH4 bydecreasing the total pressure at a constant CH4
partial pressure. For example, Figs. 5(a) and5(b) show FTIR spectra of CH4 in air before

0.20

H2O
H2O
CO2

0.00
4000 3500 3000 2500 2000 1500 1000
-1

In a CH4/N2/O2 mixture (20% O2), O2 initiallyabsorbs all of 172 nm photons and selectively
dissociates into O(1D) + O(3P), because CH4 andN2 have no absorption in this wavelength.

(2)

0.20

(b)

Wavenumber / cm
O3

O3

0.15
Absorbance(1)

It is known that O(1D) atoms are rapidly
quenched to the ground O(3P) atoms bycollisions with N2 and O2 atoms in air.
O(1D) + N2 → O(3P) + N2
(k2 = 2.6  10-11 cm3 molecule-1 s-1)17)

CH4

0.10

0.05

Effects of O(1D)O2 + hν (172 nm) → O(1D) + O(3P)(1 = 4.6 × 10-19 cm2 molecule-1)18,21)

(a)

0.15
Absorbance

700

0O(1D) + CH4 → O(3P) + CH4 (no reaction) (4)O3 / %

Concetrations of CH4, CO, and CO2 / ppm

22

0.10
CO2

0.05

H2O

H2O

CH4

0.00
4000 3500 3000 2500 2000 1500 1000
-1

Wavenumber / cm

O(1D) + O2 → O(3P) + O2
(k3 = 4.0  10-11 cm3 molecule-1 s-1)17)(3)

On the other hand, quenching reaction of O(1D)
to O(3P) by CH4 does not occur.22,23)

Fig. 5 (a) FTIR spectrum of CH4 in air at a
total pressure of 40 kPa (a) before
photoirradiation
and
(b)
after
photoirradiation for 30 min.

九州大学大学院総合理工学報告

100

3.0

2.0

CH4

60

1.5
40

1.0

20
0
0

0.5
20

40

60

80

100

0.0

Total pressure / kPa

Fig. 6. Dependence of residual amount of CH4
and O3 concentration on the total pressure
after 30 min photoirradiation.
photoirradiation and after photoirradiation for
30 min obtained by reducing the total gas
pressure from 100 kPa to 40 kPa keeping theabsolute amounts of CH4 constant (150 ppm at
100 kPa). In this condition, 52% of CH4 isdecomposed after 30 min photoirradiation. Thisvalue is 53% larger than that at 100 kPa (Fig.
2). We determined the residual amount of CH4,
[CH4]/[CH4]0, from gas analyses. Here, [CH4]0 is
the initial concentration of CH4. Figure 6 shows
the dependence of the residual amount of CH4
and the concentration of O3 on the totalpressure. It should be noted that the residual
amount of CH4 decreases from 67% to 34% withdecreasing the total pressure from 100 to 20
kPa and rapidly decreases from 34% to 3% withfurther decreasing the total pressure from 20
kPa to 10 kPa. A significant decrease in theresidual amount of CH4 with decreasing thetotal pressure from 100 kPa to 10 kPa gives
definite evidence that the O(1D) atoms play animportant role for the removal of CH4 in ourconditions.
The concentration of O3 decreases rapidly
with decreasing the total pressure, because
three-body recombination reactions (7a) and(7b) leading to O3 are greatly suppressed withdecreasing the total pressure.
O + O2 + N2  O3 + N2
(4.82  10-33 cm6 molecule-2 s-1)17)(7a)

O + O2 + O2  O3 + O2
(1.0  10-32 cm6 molecule-2 s-1)17)

(7b)

3.4

第1号

23

Effects of O3

It is known that the upper limit of the rate
constant of O3 with CH4 is extremely small at
298 K in N2 atmosphere.
O3 + CH4 → Products(1.20 × 10-21 cm3 molecule-1 s-1)17,24)(8)

We have studied the contribution of reaction (8)by isolating O3 in the reaction chamber.
Figures 7(a) and 7(b) show FTIR spectra
observed after the O3 + CH4 reaction for 1 and180 min, respectively, where reactant peaks of
CH4 and O3, a product peak of CO2, andimpurity peaks of H2O are observed. Figure 8
shows the dependence of residual amount of
CH4 and the concentration of O3 on the reaction
time. The concentration of O3 decreases from

1.5

Absorbance

2.5

(a)

O3

CH4

1.0

O3

0.5
H2O

H2 O

CO2

0.0
4000 3500 3000 2500 2000 1500 1000
-1

1.5

AbsorbanceO380

第 46 巻active species for CH4 removal in air. To
confirm this prediction, the O3 + CH4 reaction
was actually studied in the next section.
O3 concentration / ％Residual amount of CH4 / %令和 6 年度(b)

Wavenumber / cm

O3

CH4

1.0

O3

0.5

CO2
H2 O

H2O

0.0
4000 3500 3000 2500 2000 1500 1000
-1

The removal rate of CH4 increases withdecreasing the total pressure, even though theO3 concentration decreases rapidly. Thissuggests that O3 molecules will not be major

Wavenumber / cm

Fig. 7. (a) FTIR spectrum of CH4 resulting
from the O3 + CH4 reaction at reaction timesof (a) 1 min and (b) 180 min.

24

Photochemical removal of CH4 in air using a 172 nm Xe2 excimer lamp at atmospheric pressure

1.5

(a)

O2 + M

h

CH4

80

1.0

60

O3

40

0.5

20
0
0

30

60

90

120

150

Concentration of O 3 / %Residual amount of CH4 / %

100

0.0
180

Reaction time / minFig. 8. Dependence of residual amount of CH4
and concentration of O3 in the O3 + CH4
reaction at thermal energy
1.08% to 0.92% with increasing the reaction
time from 0 min to 180 min. The residual
amount of CH4 decreases more slowly from
100% to 96% after 180 min. Our present result
implies that the rate constant of reaction (8) isvery small, being consistent with the reported
very small rate constant.
3.5
Possible oxidation mechanism of CH4
under 172 nm photolysis in air
O(3P), O(1D), and O3 are formed by 172 nm
photolysis of O2 in air as shown in Scheme 1(a).
Among them, we found that the metastableO(1D) atoms with an excitation energy of 1.967
eV play a significant role for the removal of CH4
in air under our experimental conditions. The
contributions of 172 nm photons and O(3P) are
negligible and that of O3 is very small. Active
O(1D) atoms arise exclusively from the direct
172 nm photolysis of O2 (1) in the initial stage.
The rate constants of the O(1D) + CH4
reaction leading to CH3, CH2O, and CH3O havebeen measured as.
O(1D) + CH4  CH3 + OH(9a) 66%(k9a = 1.13  10-10 cm3molecule-1s-1)17)
 CH2O + H2(9b) 4.4%(k9b =7.51  10-12 cm3molecule-1s-1)17)
 CH3OH(9c) 29%(k9c = 4.98  10-11 cm3molecule-1s-1)17)
The branching fractions of the above three
product channels indicate that major initial
products are CH3 + OH. On the basis of known
oxidation reactions of CH3 and their rate
constants,17,25) major two-body and three-bodyreaction pathways from CH3 to CO and CO2 are

O(3P)

O2

h

O3

+M

h

O(1D)
h
O(3P)

(b)
O(1D)

CH4

M = N2, O2

OH

CH3

O2 + M

CH3O2
O(3P), OH

O2

CH3O
O(3P), O2

CH2O
O(3P), OH

HCO

O2

CO

O+ M
OH

CO2

Scheme 1. (a) photolysis of O2 and O3 under
172 nm VUV irradiation in air. (b) majoroxidation processes of CH4 under 172 nm
VUV photolysis in air.
shown in Scheme. 1(b). Although only CO
molecules are detected as an intermediate of
the final product CO2 in this study, short lived
CH3O2, CH3O, CH2O, and HCO will be formed
as intermediates.
OH radicals formed in process (9a) can reactwith CH4, and CH3 + H2O are formed.
OH + CH4 → CH3 + H2O(10)(k10 = 6.38  10-15 cm3 molecule-1 s-1)17)
Since the rate constant of the OH + CH4
reaction (10) is much smaller than that of theO(1D) + CH4 reaction (6), its contribution toinitial decomposition of CH4 to CH3 will besmall. As shown in Scheme 1(b), OH radicals
can take part in the oxidation of someintermediate species. Therefore, they may takepart in the oxidation of some intermediates.
O(3P) atoms and O2 molecules do notparticipate in the initial decomposition of CH4.
However, they can contribute to the oxidationof such intermediates as CH3, CH3O2, CH3O,
CH2O, HCO, and CO as shown in Scheme 1(b).
Present experimental data and reported kinetic
data17) suggest that O3 molecules are lessimportant than O(3P) and O2 in the oxidation of
CH4 under 172 nm photoirradiation. Although
172 nm photoabsorption and photodissociation
data of CH3, CH2O2, CH3O, CH2O, and HCO
intermediates have not been known, direct
photodissociation processes may also take partin the oxidation processes of above
intermediates shown in Scheme 1(b).

令和 6 年度

4.

九州大学大学院総合理工学報告

Summary and Conclusion

The photolysis of methane by using 172 nm
Xe2 excimer lamps was studied to develop a
new simple photochemical removal method of
CH4 without using any expensive catalysts atroom temperature. No photolysis of CH4 wasobserved in N2, whereas CH4 was oxidized to
CO and CO2 in air. In the presence of O2,
O(3P,1D) and O3 can be active species for theremoval of CH4. Effects of O(1D) were examinedby increasing the [O(1D)]/[O(3P)] ratio at low
total pressures. The contribution of O3 wasdetermined by observing CH4 removal by theO3 + CH4 reaction using the same apparatus.
Results showed that O(1D) is the responsible
active species under the 172 nm photolysis of
CH4 and the contributions of O(3P) and O3 areunimportant in the initial stage of the CH4
decomposition. The removal rate of CH4 byusing the side-on lamp was about 6 times fasterthan that using the head-on lamp because oflarger irradiation volume in the reaction
chamber. Photochemical removal of CH4 ismore difficult than that of C6H6, CH3CHO, andC2H3CHO,12,13,16) because its oxidation by theO(3P) + CH4 and O3 + CH4 reactions are very
slow and no direct photolysis of CH4 occurs by
172 nm photons. These fundamental data
provide valuable information for the design ofpractical removal apparatus CH4 under VUVphotolysis in air.

4)
5)
6)

7)
8)

9)

10)

11)
12)

13)

14)

15)
16)

18)

This work was supported by NEDO
(2008−2009), and City Area Project from
Fukuoka Prefecture (2009).

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